1. Field of the Invention
The present invention relates to magnetic memories, more particularly to magnetic memories incorporating synthetic antiferromagnet free layers that are composed of multiple ferromagnetic layers separated by non-magnetic layers.
2. Description of the Related Art
The magnetic memory (MRAM) is one of the promising non-volatile semiconductor storage devices. A memory cell of the magnetic memory is typically composed of a magnetoresistance element including a fixed magnetic layer and a free magnetic layer separated by a spacer layer; data is stored as the direction of the magnetization of the free magnetic layer. Reading data from a memory cell is achieved on the basis of the change in the resistance of the magnetoresistance element resulting from the tunneling magnetoresistance (TMR) effect or the Giant magnetoresistance (GMR) effect. A thin insulating layer is used as the spacer layer for a magnetic memory based on the TMR effect, while a thin non-magnetic layer is used for a magnetic memory based on the GMR effect.
One of the issues of magnetic memories is selectivity of memory cells during write operations. A typical magnetic memory achieves writing data into a selected memory cell through developing write currents on selected word and bit lines associated with the selected memory cell. Ideally, data is not written into a half-selected memory cell associated with only one, not both of the selected word and bit lines; however, data may be undesirably written into a half-selected memory cell through variances of the magnetic fields for inversing the magnetization of the free magnetic layer within the memory cell. Avoiding such undesirable data write operations is one of the important issues for developing a commercially-available magnetic memory.
In order to improve the selectivity of memory cells, Savtchenku et al. discloses a technique that adopts synthetic antiferromagnets for free magnetic layers in U.S. Pat. No. 6,545,906; it should be noted that the synthetic antiferromagnet designates a stacked structure composed of two or more ferromagnetic layers separated by one or more intermediate non-magnetic layers, designed so that two adjacent ferromagnetic layers are antiferromagnetically coupled through exchange interaction. Those skilled in the art would appreciate that the antiferromagnetic coupling based on the exchange interaction is achieved by appropriately selecting the thickness and/or the material of the intermediate non-magnetic layer.
An advantage of the use of a synthetic antiferromagnet for a free magnetic layer is that the synthetic antiferromagnet is stable against the external magnetic field. This result from the fact that total magnetic moment of the synthetic antiferromagnet is ideally allowed to be zero; although the term “synthetic antiferromagnet” may generally mean to that which has a certain resultant magnetic moment, the term “synthetic antiferromagnet” in this specification is intended to include that whose the total magnetic moment is zero.
The stability of the synthetic antiferromagnet against the external magnetic field may seem to cause difficulty in data write operations into the free magnetic layer; however, such difficulty is resolved by a special data write operation, referred to as toggle writing, hereinafter, as disclosed by Savtchenku et al.
FIG. 1 is a section view illustrating the structure of an MRAM memory array adapted to toggle writing disclosed by Savtchenku et al., and FIG. 2 is a plan view illustrating the structure of the MRAM memory array. As shown in FIG. 1, the MRAM memory array is composed of a word line 101, a bit line 102, and a memory cell 103 connected between the word line 101 and the bit line 102; those skilled in the art would understand that FIG. 1 partially illustrates the structure of the MRAM memory array, which incorporates a plurality of word lines, bit lines, and memory cells. The memory cell 103 incorporates a fixed magnetic layer 104, a free magnetic layer 105, and a tunnel barrier layer 106 disposed therebetween. The fixed magnetic layer 104 and the free magnetic layer 105 is each composed of a synthetic antiferromagnet; the fixed magnetic layer 104 is formed of a pair of ferromagnetic layers 111 and 112 separated by a non-magnetic layer 113, and the free magnetic layer is formed of a pair of ferromagnetic layers 114 and 115 separated by a non-magnetic layer 116. Hereinafter, magnetizations of the ferromagnetic layers 114 and 115 are denoted by symbols M1, and M2, respectively, and a resultant magnetization of the free magnetic layer 105 is referred to as a symbol MR.
As shown in FIG. 2, the easy axes of the ferromagnetic layers 114 and 115 are directed at an angle of 45° with respect to the word and bit lines 101 and 102. The magnetization M1 of the ferromagnetic layer 114 is directed antiparallel to the magnetization M2 of the ferromagnetic layer 115. FIGS. 3 and 4 illustrate a procedure of the toggle writing disclosed by Savtchenku et al. As shown in FIG. 2, an x-y coordinate system is defined for easy understanding in the following. The x-axis is defined along the word line 101, while the y-axis is defined along the bit line 102.
Referring to FIG. 3, the toggle writing operation begins with developing a current IWL directed in the positive x-direction on the word line 101 at a time t1. As shown in FIG. 4, the current IWL induces a magnetic field HWL in the positive y-direction, and the developed magnetic field HWL slightly rotates the magnetizations M1 and M2 of the ferromagnetic layers 114 and 115 toward the positive y-direction. The rotation angles of the magnetizations M1 and M2 are determined so that the resultant magnetization MR is directed in parallel with the +y direction.
As shown in FIG. 3, this is followed by developing a current IBL directed in the +y direction on the bit line 102 at a time t2. As shown in FIG. 4, developing the current IBL results in developing a synthetic magnetic field HWL+HBL directed at an angle of 45° with respect to the x and y axes. The synthetic magnetic field HWL+HBL rotates the magnetizations M1 and M2 in a clockwise direction so that the resultant magnetization MR thereof is directed in parallel with the synthetic magnetic field HWL+HBL.
The current IWL on the word line 101 is then terminated at a time t3. The termination of the current IWL results in only the magnetic field HBL, directed in the positive x-direction, is effected on the ferromagnetic layers 114 and 115. The magnetic field HBL further rotates the magnetizations M1 and M2 so that the resultant magnetization MR thereof is directed in parallel with the magnetic HBL.
Finally, the current IBL on the bit line 102 is then terminated at a time t4. The termination of the current IBL removes the magnetic field from the magnetizations M1 and M2, allowing the magnetizations M1 and M2 to be directed in parallel with the easy axes. As a result, the magnetizations M1 and M2 are reversed into the opposite directions to the original directions thereof. It should be noted that the toggle writing reverses the magnetizations M1 and M2, independently of the original directions thereof.
For the practical application, it is important to reduce the magnetic field necessary for reversing the magnetizations within the free magnetic layer 105 (which may be referred to as the spin-flop field, hereinafter), that is, the current level of the write currents developed on the word and bit lines 101 and 102.
Engel discloses a technique for reducing the current level necessary for “toggling” the magnetizations within the synthetic antiferromagnet, in U.S. Pat. No. 6,714,446. As shown in FIG. 5, the Engel's magnetic memory is composed of a free magnetic layer 105 which includes a synthetic antiferromagnet 114 formed of ferromagnetic layers 114a and 114b separated by a non-magnetic layer 114c; a synthetic antiferromagnet 115 formed of ferromagnetic layers 115a and 115b separated by a non-magnetic layer 115c, and a non-magnetic layer 116 disposed between the synthetic antiferromagnets 114 and 115.
Although U.S. Pat. No. 6,545,906 disclosed that there are no upper limits of magnetic fields for achieving the toggle writing, an inventors' study has proved that there are upper limits of magnetic fields for successfully achieving the toggle writing. More specifically, the magnetizations of the ferromagnetic layers within the free magnetic layer are aligned in the same direction, when excessively large magnetic fields are applied to the free magnetic layer along the word and bit lines; directing the magnetizations within the free magnetic layer in the same direction causes uncertainty of the directions of the magnetizations after the removal of the magnetic fields, unsuccessfully completing the toggle writing. Improvement in the stability of the toggle writing requires increasing the upper limits of magnetic fields for successfully achieving the toggle writing.
There is a need for increasing the upper limits of magnetic fields for successfully achieving the toggle writing, and thereby improving the operation range of the magnetic memory.